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Re: [Qemu-block] [PATCH] block-backend: allow flush on devices with open

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH] block-backend: allow flush on devices with open tray
Date: Tue, 14 Jun 2016 18:13:00 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 14.06.2016 17:54, John Snow wrote:
> On 06/14/2016 09:19 AM, Max Reitz wrote:
>> On 10.06.2016 23:59, John Snow wrote:
>>> If a device still has an attached BDS because the medium has not yet
>>> been removed, we will be unable to migrate to a new host because
>>> blk_flush will return an error for that backend.
>>> Replace the call to blk_is_available to blk_is_inserted to weaken
>>> the check and allow flushes from the backend to work, while still
>>> disallowing flushes from the frontend/device model to work.
>>> This fixes a regression present in 2.6.0 caused by the following commit:
>>> fe1a9cbc339bb54d20f1ca4c1e8788d16944d5cf
>>> block: Move some bdrv_*_all() functions to BB
>>> Signed-off-by: John Snow <address@hidden>
>>> ---
>>>  block/block-backend.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> I'm still not sure we shouldn't do the same for blk_{co,aio}_flush(). I
>> guess you exclude them here because you specifically want to fix the
>> issue mentioned in the commit message, but then we could just make
>> blk_flush_all() ignore an -ENOMEDIUM.
> Yeah, I didn't investigate the full path. Just making the minimal fixes.
> Is there a concern that this may still leave certain pathways broken
> when the CDROM tray is open?
> I don't know of any immediately without digging again.
>> I personally think we should make all blk_*flush() functions use
>> blk_is_inserted() instead of blk_is_available(). As we have discussed on
>> IRC, there are probably not that many cases a guest can flush a medium
>> in an open tray anyway (because the main use case are read-only
>> CD-ROMs), and even if so, that wouldn't change any data, so even if the
>> guest can actually flush something on an open tray, I don't think anyone
>> would complain.
>> Max
> I have difficulty making pragmatic arguments when purity is at stake,
> but I've already wandered outside of my device model, so I will defer to
> your judgment.
>>> diff --git a/block/block-backend.c b/block/block-backend.c
>>> index 34500e6..d1e875e 100644
>>> --- a/block/block-backend.c
>>> +++ b/block/block-backend.c
>>> @@ -1122,7 +1122,7 @@ int blk_co_flush(BlockBackend *blk)
>>>  int blk_flush(BlockBackend *blk)
>>>  {
>>> -    if (!blk_is_available(blk)) {
>>> +    if (!blk_is_inserted(blk)) {
>>>          return -ENOMEDIUM;
>>>      }
> Is this a NACK unless I attempt to address the wider design issue?

I just don't see a point in using blk_is_inserted() here but
blk_is_available() in the other blk_*flush() functions. If
blk_is_inserted() is correct for blk_flush(), it should be correct for
blk_co_flush() and blk_aio_flush(), too.

Maybe I should emphasize that I decided between is_available() and
is_inserted() basically on what felt right to me. There's not really
that much research behind it, so changing it is completely fine.


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